Use modern type trait implementations when available.

Teach libcxx to stop using various deprecated __has_* type traits, in favor of
the ("modern", C++11 era) __is_* type traits.

This is mostly just a simplification, but fixes at least one bug: _Atomic T
should be considered trivially-destructible, but is not considered to be POD by
Clang, and __has_trivial_destructor is specified in the GCC documentation as
returning false for non-POD non-class types.

Differential Revision: https://reviews.llvm.org/D48292

llvm-svn: 359159